Purpose: To measure the neural responses underlying visual masking. We tested the hypothesis that perception of a target stimulus is impaired because the mask suppresses the neural response to the target.

Methods: Subjects performed a contrast discrimination task on a target grating which was superimposed on a 2D noise mask. The stimulus was restricted to an annulus (0.75 – 2.25 deg eccentricity), and the contrasts of the two components were independently varied. Using a block-alternation design, the fMRI responses, in early visual areas, to four target contrasts (5%, 10%, 20%, and 40%), each superimposed on three mask contrasts (0%, 25%, 50%), were measured with a 3T GE magnet. Retinotopic visual areas were mapped using standard techniques, and the analysis was restricted to the subregion that responded to the annular stimulus.

Results: For a given mask contrast, the fMRI response increased with increasing target contrast. For a given target contrast, the fMRI response decreased with increasing mask contrast. Using a model to link the fMRI responses to the contrast discrimination thresholds, we found that the suppression predicted from the discrimination thresholds was similar to the observed fMRI suppression. In a control experiment, we used the same high contrast mask, but eliminating the psychophysical masking by temporally offsetting the mask. In these experiments, we found no suppression of the fMRI response.

Conclusion: The pooled activity of neurons in early visual cortex potentially underlies the perceptual phenomenon of visual masking.